De-ramp or “stretch” processing is commonly used in high-performance, wide-bandwidth radar systems to obtain fine range resolution without requiring unreasonably high or unobtainable Analog to Digital Converter (ADC) sampling frequencies and the resulting data processing burden. Systems that employ linear-FM “chirp” waveforms typically employ pulse compression by de-ramping, which mixes (convolves) the received signal (radar echo) with a reference chirp waveform. The linear-FM chirp is completely removed by the de-ramp mixer because the reference chirp rate is matched to the chirp rate of the received radar echo signal. A conventional example of this is shown graphically in the frequency-vs.-time plot of FIG. 1. The resulting Intermediate Frequency (IF) bandwidth at the de-ramp output is usually a fraction of the input bandwidth. This bandwidth compression permits an ADC to meet the needs of a radar system having an RF bandwidth that is considerably larger than the Nyquist (half sample rate) bandwidth of the ADC.
In wide bandwidth, high dynamic range radar modes such as Synthetic Aperture Radar (SAR) and Moving Target Indication (MTI), point-targets (those returns that occupy a single range “bin”) are often prevalent. With conventional de-ramp processing, the point targets result in corresponding single-frequency tones (sinusoids) at the ADC input. It is well known that pure sinusoids combined with ADC nonlinearities (e.g., Integral Non-Linearity or INL) result in undesired spurious signals with maximum peak spectral amplitude. In SAR systems, the result is false targets in the range/Doppler imagery. In MTI systems, the result is false detections. Other noise sources (described hereinbelow) are also known to generate unwanted spurious signals and corresponding false results.
FIGS. 2 and 3 graphically illustrate an example of the result of conventional de-ramp processing using the reference chirp of FIG. 1. FIGS. 2 and 3 show the desired output signal together with the aforementioned undesired spurs. FIG. 2 is a frequency-vs.-time plot, and FIG. 3 is an amplitude-vs.-time plot. As shown, spurs due to ADC nonlinearities and other noise sources manifest as harmonically related sine wave tones in addition to the sine wave tone of the desired output signal. The result is a minimization of spurious-free dynamic range (i.e., the spurs extend above the noise floor in FIG. 3) and correspondingly reduced system performance.
It is desirable in view of the foregoing to provide for processing techniques that reduce the aforementioned incidences of false results in radar systems.